1. Field of the Invention
The present invention relates to an electro-optic probe, which observes waveforms of test signals from the polarization state of an incident light when incident to an electro-optic crystal which is coupled with an electric field generated by the test signal, and particularly relates to an improved electro-optic probe.
2. Background Art
A waveform of a test signal can be measured from the polarization state of a laser light when the laser light enters an electro-optic crystal which is coupled with an electric field generated by the test signal. A very high time resolution can be obtained by use of a laser pulse and by executing sampling. Hereinafter, the electro-optic sampling oscilloscope is described which is formed by the use of such an electro-optic probe.
The electro-optic sampling oscilloscope (hereinafter called EOS oscilloscope) has following advantages over the conventional sampling oscilloscope using electric probes:
(1) Measurement is easy, because a ground line is not necessary during measurement.
(2) Since the metal pin disposed at the top of the electro-optic prove is insulated from the measuring circuit, a high input impedance is provided, which results in eliminating factors that disturb the conditions of the test point.
(3) The use of the light pulse allows carrying out wide band measurement reaching to the GHz order.
The structure of the conventional electro-optic probe used in the measurement of signals by the EOS oscilloscope will be described with reference to FIG. 3. In the electro-optic probe shown in FIG. 3, the numeral 1 denotes a probe head made of an insulator, and a metal pin 1a is inserted in the probe head 1. The reference numeral 2 denotes an electro-optic element and a reflecting film 2a is formed on an outside surface of the electro-optic element 2, to which the metal pin 1a contacts. The reference numerals 3 and 8 denote collimating lenses and the numeral 4 denotes a quarter-wave plate. The numerals 5 and 7 denote polarizing beam splitters. The numeral 6 denotes a Faraday element which rotates the plane of polarization 45 degrees. The numeral 9 denotes a laser diode for emitting a laser beam in response to a control signal output by an EOS oscilloscope body (not shown), which receives the output of the probe after converting the laser beam into electric signals. The numeral 12 denotes a probe body, and the numeral 13 denotes an optical isolator comprising the quarter-wave plate 4, polarizing beam splaitters 5 and 7, and the Faraday element 6.
Next, the optical path of the laser light emitted from the laser diode 9 will be described with reference to FIG. 3. The path of the laser light is represented by the reference symbol A.
The laser beam emitted from the laser diode 9 is collimated into a parallel beam by the collimating lens 8, and enters the electro-optic element 2, after rectilinearly advancing through the polarizing beam splitter 7, the Faraday element 6, and the polarized beam splitter 5, and further passing the quarter-wave plate 4 and condensed by the collimating lens 3. The light beam input into the electro-optic element 2 is reflected by the reflecting film 2a formed at the end surface of the electro-optic element 2 facing to the metal pin 1a. 
The reflected laser beam enters the photodiode 16, after being collimated into a parallel beam by the collimating lens 3, passing the quarter-wave plate 4, and a part of the laser beam is reflected by the polarizing beam splitter 5. The part of the laser beam transmitted by the polarizing beam splitter 5 enters the photodiode 11 after being reflected by the polarizing beam splitter 5.
The rotation angles of the quarter-wave plate 4 are adjusted such that the intensities of two laser beams entering the two photodiodes becomes identical.
Hereinafter, the operation of measurement by the use of the electro-optic probe shown in FIG. 3 is described.
When the metal pin 1a contacts a test point, an electric field is generated at the metal pin 1a and the electric field propagates to the electro-optic element 2, which results in causing a change of the refractive index of the electro-optic element 2 due to the Pockels effect. When the laser beam emitted by the laser diode 9 enters and propagates through the electro-optic element after the refractive index of the electro-optic element changes, the polarization state of the laser light changes. The laser beam having the thus changed polarization state is introduced into the photodiodes 10 and 11 and is converted into an electric signal after being reflected by the reflecting film 2a and converted into the electric signals.
The change of the voltage applied to the measuring point is reflected as the change of the polarization state of the laser light by the electro-optic element 2, and the change of the polarization state is detected by the difference between the output from the photodiodes 10 and 11. Thus, the electric signal applied to the metal pin 1a can be measured as the difference betweeen the output of the photodiodes 10 and 11.
In the electro-optic probe shown above, the electric signals obtained from these photodiodes are input into an oscilloscope for processing, but it is possible to measure signals by connecting a controller for controlling the signal measurement between these photodiodes 10 and 11 and a measuring device such as a real time-type oscilloscope. Thereby, wide band measurement is facilitated by the use of the electro-optic probe.
However, in the conventional electro-optic probe, as shown by the symbol B in FIG. 3, the laser beam, regularly reflected at the surface of the quarter-wave plate 4a enters the photodiode 10 after being reflected by the polarizing beam splitter 5. This laser beam constitutes a noise and degrades the S/N ratio after being converted into the electric signal when observed by the oscilloscope. This noise beam is generated not only from the surface 4a of the quarter-wave plate 4, but also from the surface 5a of the polarizing beam splitter 5 as shown by the symbol C in FIG. 3 as well as from other optical components.
Although the reflectance of the optical components can be reduced by applying the antireflection coating, it is not possible to minimize the reflectance to null, and the costs of optical components may rise by applying the antireflection coating.
It is therefore an objective of the present invention to provide an electro-optic probe which is capable of reducing unnecessary reflected light and capable of improving the S/N ratio.
According to the first aspect of the present invention, an electro-optic prove comprises: a laser diode for emitting a laser beam based on control signals of an oscilloscope body; a first lens for collimating the laser beam into a parallel beam; a second lens for condensing said parallel beam; an electro-optic element having a reflecting film on one end surface; an isolator for isolating the reflected laser beam reflected by said reflecting film after the laser beam emitted by said laser diode passes to the reflecting film; and a plurality of photodiodes for converting the reflected laser beam after being separated by said isolator; wherein, optical components constituting said isolators are disposed inclining to the optical axis at an angle such that the light beams reflected on surfaces of said optical components by regular reflection does not enter said photodiodes.
According to the second aspect, in an electro-optic probe according to the first aspect, said plurality of photodiodes and said laser diode are connected to an electro-optic sampling oscillator and said laser diode emits a pulsed laser based on the control signal generated by said electro-optic sampling oscillator.
According to the third aspect, in an electro-optic probe according to the first aspect, said laser diode emits a continuous laser beam.
According to the fourth aspect, in an electro-optic probe according to the first aspect, said inclination angle of optical components is set within a range from an angle formed by an optical path from said optical component to a light receiving element in said photodiode and the diameter of said light receiving element to an angle allowable for the optical component to maintain the transmittance thereof.